Method and system for providing path-level access control for structured documents stored in a database

ABSTRACT

An improved method and system for providing path-level access control to a structured document in a collection stored in a database, where the structured document includes a plurality of nodes is disclosed. The method includes the steps of providing an access control policy for the collection, where the access control policy comprises a plurality of access control rules, generating a path for each node of the plurality of nodes in the document, and generating for each path associated with a node a corresponding value expression based on at least one access control rule of the plurality of access control rules. According to the method and system of the present invention, the corresponding value expression is utilized during access control evaluation to determine whether a user is allowed to access a node in the structured document.

FIELD OF THE INVENTION

The present invention relates generally to computer implemented databasesystems and, more particularly, to a method and system for providingpath-level access control for structured documents stored in a databasesystem.

BACKGROUND OF THE INVENTION

Structured documents are documents which have nested structures.Documents written in Extensible Markup Language (XML) are structureddocuments. XML is quickly becoming the standard format for deliveringinformation over the Internet because it allows the user to design acustomized markup language for many classes of structure documents. Forexample, a business can easily model complex structures such as purchaseorders in XML form and send them for further processing to its businesspartners. XML supports user-defined tabs for better description ofnested document structures and associated semantics, and encourages theseparation of document content from browser presentation.

As more and more businesses present and exchange data in XML documents,database management systems (DBMS) have been developed to store, queryand retrieve these documents which are typically stored on direct accessstorage devices (DASD), such as magnetic or optical disk drives forsemi-permanent. Some DBMSs, known as relational databases, store andquery the documents utilizing relational techniques, while other DBMSs,known as native databases, store the documents in their native formats.XML documents are typically grouped into a collection of similar orrelated documents. Thus, for example, a group of purchase orders canform a collection.

Once a collection of documents is stored in the database, relational ornative, it is potentially available to large numbers of users.Therefore, data security becomes a crucial concern. In particular, theDBMS must be able to control, i.e., deny or grant, access to the data bythe user. In a conventional relational DBMS where the data is stored inrows and columns in tables, security is generally directed to the tablelevel, i.e., access to a table is controlled. While this may besufficient for relational data, it is inadequate for controlling accessto a collection of XML documents because an XML document stored in thedatabase contains information that is much more diverse than data storedin rows in tables.

Access control for XML documents is fine-grained, that is, access toeach node in an XML document is controlled. The term “node” is used inthe DOM-sense, which is a standard XML construct well known to thoseskilled in the art. In that construct, the XML document is representedby a plurality of nodes that form a hierarchical node tree. Each node ofthe XML document is identified by a path that defines a hierarchicalrelationship between the node and its parent node(s). Thus, fine-grainedaccess control to the nodes of an XML document is referred to aspath-level access control.

For example, if an administrator wanted to limit access to a “salary”node in all documents in a collection “all_employees,” the administratorwould generate the following statement:

-   Deny read access on “/employee/salary” in collection “all_employees”    to group non-managers    This statement would deny access to all salary nodes with path    “/employee/salary” in all documents in collection “all_employees.”    This type of statement is referred to as an access control rule. A    set of access control rules directed to a collection of documents is    referred to as an access control policy.

While it is possible to perform path-level access control evaluation byutilizing access control rules, such evaluation is relatively expensivebecause the DBMS must evaluate each access control rule to determinewhether a user should be granted or denied access to data in a node.This process becomes prohibitive when the number of access control rulesin a policy increases. Nevertheless, the alternative, i.e.,coarse-grained access control or table level access control, isunacceptable.

Accordingly, a need exists for an improved method and system forproviding path-level access control for structured documents stored in adatabase. The method and system should be integrated (or capable ofbeing integrated) with an existing database system in order to use theexisting resources of the database system. The present inventionaddresses such a need.

SUMMARY OF THE INVENTION

The present invention is directed to an improved method and system forproviding path-level access control to a structured document in acollection stored in a database, where the structured document comprisesa plurality of nodes. The method includes providing an access controlpolicy for the collection, where the access control policy comprises aplurality of access control rules, generating a path for each node ofthe plurality of nodes in the document, and generating for each pathassociated with a node a corresponding value expression based on atleast one of the plurality of access control rules. According to themethod and system of the present invention, the corresponding valueexpression is utilized during access control evaluation to determinewhether a user is allowed to access a node in the structured document.

Through the aspects of the present invention, an Access Controlmechanism in the DBMS receives the access control policy for thestructured document in the collection and generates for each of thepaths associated with the nodes in the document a value expression. Thevalue expression is an executable statement which describes the accesscontrol rule for that path, i.e., who is granted or denied access todata in that path. Because the value expression is easier to processthan the access control policy, access control evaluation and processingis more efficient and faster.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary computer environment for usewith the present invention.

FIG. 2 is a block diagram of the Access Control mechanism according tothe preferred embodiment of the present invention.

FIG. 3 is a flowchart illustrating a process for providing path-levelaccess control to structured documents stored in a database according toa preferred embodiment of the present invention.

FIG. 4 is a flowchart illustrating a process for generating valueexpressions according to a preferred embodiment of the presentinvention.

FIG. 5 illustrates a condition table according to a preferred embodimentof the present invention.

FIG. 6 illustrates a path table according to a preferred embodiment ofthe present invention.

FIG. 7 is a flowchart illustrating a process for utilizing valueexpressions during access control evaluation according to a preferredembodiment of the present invention.

DETAILED DESCRIPTION

The present invention relates generally to computer implemented databasesystems and, more particularly, to an improved method and system forproviding path-level access control for structured documents stored in adatabase. The following description is presented to enable one ofordinary skill in the art to make and use the invention and is providedin the context of a patent application and its requirements. Variousmodifications to the preferred embodiment and the generic principles andfeatures described herein will be readily apparent to those skilled inthe art. For example, the following discussion is presented in thecontext of a DB2® database environment available from IBM® Corporation.It should be understood that the present invention is not limited to DB2and may be implemented with other database management systems. Thus, thepresent invention is to be accorded the widest scope consistent with theprinciples and features described herein.

According to a preferred embodiment of the present invention, astructured document is parsed into a plurality of nodes that form a nodetree. Each node is then associated with a path that describes the node'shierarchical relationship to its parent node(s). A translator receivesan access control policy for the structured document that comprises atleast one access control rule, and generates for each path acorresponding value expression based on the access control policy. Thevalue expression is a simple statement granting or denying access to thenode associated with the path. In a preferred embodiment, the valueexpressions for the structured document are compiled resulting in a setof mini-plans that can be efficiently executed at runtime by the DBMS.

When a user issues a query on a certain node in the structured document,the DBMS navigates to the certain node and evaluates the correspondingvalue expression, as opposed to the access control rule(s) in the accesscontrol policy, to determine whether the user is authorized to accessthe certain node. Because the value expression is less complex than theaccess control rule(s), evaluation by the DBMS is more efficient andfaster. Moreover, according to another preferred embodiment of thepresent invention, access control is evaluated during runtime, asopposed to compile time, thereby allowing an administrator to change theaccess control policy at runtime without having to recompile the query.

To describe further the present invention, please refer to FIG. 1, whichis an exemplary computer environment for use with the present invention.In FIG. 1, a typical distributed computer system utilizes a network 103to connect client computers 102 executing client applications to aserver computer 104 executing software and other computer programs, andto connect the server computer 104 to data sources 106. These systemsare coupled to one another by various networks, including LANs, WANs,and the Internet. Each client computer 102 and the server computer 104additionally comprise an operating system and one or more computerprograms (not shown).

The server computer 104 uses a data store interface (not shown) forconnecting to the data sources 106. The data store interface may beconnected to a database management system (DBMS) 105, which supportsaccess to the data store 106. The DBMS 105 can be a relational databasemanagement system (RDBMS), such as the DB2® system developed by IBMCorporation, or it also can be a native XML database system. Theinterface and DBMS 105 may be located at the server computer 104 or maybe located on one or more separate machines. The data sources 106 may begeographically distributed.

The DBMS 105 and the instructions derived therefrom are all comprised ofinstructions which, when read and executed by the server computer 104cause the server computer 104 to perform the steps necessary toimplement and/or use the present invention. While the preferredembodiment of the present invention is implemented in the DB2® productoffered by IBM Corporation, those skilled in the art will recognize thatthe present invention has application to any DBMS, whether or not theDBMS 105 is relational or native. Moreover, those skilled in the artwill recognize that the exemplary environment illustrated in FIG. 1 isnot intended to limit the present invention, and that alternativeenvironments may be used without departing from the scope of the presentinvention.

According to the preferred embodiment of the present invention, the DBMS105 includes access control policies 107 that are authored by anadministrator 108 or some other authorized personnel. Each accesscontrol policy 107 describes the security rules pertaining to datastored in the database. The DBMS 105 also comprises an Access Controlmechanism 200 that provides path-level access control to structureddocuments stored on disk. Storing data “on disk” refers to storing datapersistently, for example, in the data store 106.

FIG. 2 is a block diagram of the Access Control mechanism 200 accordingto the preferred embodiment of the present invention. The Access Controlmechanism 200 comprises a path table generator 202, a translator 300 anda path table 204. Each component will be described in further detail inconjunction with FIG. 3.

FIG. 3 is a flowchart illustrating a method for providing path-levelaccess control to structured documents stored in the database accordingto a preferred embodiment of the present invention. In step 304, theAccess Control mechanism 200 receives or retrieves an access controlpolicy 107 pertaining to a collection of structured documents. In apreferred embodiment, the Access Control mechanism 200 receives thecontrol policy 107 immediately after the administrator 108 has writtenand validated it. In another preferred embodiment, the control policy107 can be stored in a repository in the DBMS 105, and retrieved by theAccess Control mechanism 200 at a time after authorship.

The access control policy 107 for the collection comprises a pluralityof access control rules. Each access control rule typically defines asubject to which the rule applies, an action and a path. The subject canbe a user's name or a group of users. The action can be, but is notlimited to, a read, an update, a create or a delete action. The pathidentifies the node to which the rule applies. For example, thefollowing access control rule:

-   -   </bib/book/title, {Murata}, +read>        provides that the user Murata is allowed to read information at        the title element node described by the path “/bib/book/title.”        The access control rule can also include predicates such that        access to a particular node can be data-dependent. The access        control rules are presumably syntactically correct and logically        valid when they are received by the Access Control mechanism        200. Alternatively, the Access Control mechanism 200 can be        configured to evaluate the access control policy 107 and        validate it.

Referring again to FIG. 3, once the access control policy 107 has beenreceived by the Access Control mechanism 200, the path generator 202generates a path for each node in a node tree representing a structureddocument in the collection in step 306. As stated above, it is wellknown to those skilled in the art that a structured document can berepresented as a plurality of nodes forming a hierarchical node tree.Thus, for example, suppose a structured document, S1, is as follows:

<Questionnaire>    <Name> Alice </Name>    <Questions>       <Q1> Yes</Q1>       <Q2> No </Q2>    </Questions> <Questionnaire>The node tree would comprise the following element nodes:

The path to element node Q1 is:

-   -   /Questionnaire/Questions/Q1

Referring again to FIG. 3, once the paths have been generated, theaccess control policy 107 and paths are passed to the translator 300,where in step 308, a value expression for each path is generated. In apreferred embodiment, the value expression for a path is an executablestatement regarding who, if anyone, has access to the node associatedwith the path. The value expression represents a combination of allaccess control rules that affect the node. The process by which thetranslator 300 generates the value expressions is illustrated in FIG. 4.

Referring now to FIG. 4, the process begins by normalizing each of theaccess control rules that make up the access control policy 107 in step402. In a preferred embodiment, each access control rule is transformedinto a normalized rule format that includes a head, a path expression,and a condition. The head indicates whether an action is granted ordenied, the path expression describes the path associated with the node,and the condition indicates to whom the rule applies and under whatcircumstances. For example, the following access control rule:

-   -   </bib/book[@title=“security”], GROUP Admin, +read>        is transformed into:

-   (grant_read, ‘/bib/book’, equal($Group, Admin) &    xpath(/bib/book[@title=“security”])    The above format is called a normalized rule. After this step is    finished, a set of normalized access control rules is generated.

Next, in step 404, the translator 300 generates and populates acondition table. FIG. 5 illustrates a condition table 500 according to apreferred embodiment of the present invention. The condition table 500comprises a ConditionID column 502 and a Condition Expression column504. The Condition Expression column 504 contains the third argument ofeach normalized rule, i.e., the condition, while the ConditionID column502 contains an identifier associated with the condition. The translator300 replaces the third argument of each normalized rule with theConditionID corresponding to the condition to form a modified normalizedrule.

Referring again to FIG. 4, after the condition table has been generated,the translator 300 propagates each modified normalized rule through eachpath generated by the path generator 202 (in step 306 of FIG. 3) in step406. Propagation refers to how an access control rule associated with aparticular node affects access to ancestor and descendant nodes. Ingeneral, propagation has two modes: up and down. Propagation-up refersto when an access control rule associated with a particular nodepropagates upward toward its ancestor nodes, while propagation-downrefers to when an access control rule propagates downward toward itsdescendant nodes. Whether an access control rule propagates up or downdepends on a set of propagation rules. The following table illustrates aset of propagation rules according a preferred embodiment of the presentinvention:

TABLE 1 Action Effect Mode Semantics Read Grant Down Rule specified toany node propagates down. Up Every GRANT for read-access propagates upto all ancestor elements (but not including the attributes and textnodes of these ancestor elements). Deny Down Rule specified to any nodepropagates down. Up No propagation Update Grant Down Every GRANT forupdate-access propagates down to all the attributes and text nodes. UpNo propagation Deny Down No propagation Up No propagation Create GrantDown No propagation Up No propagation Deny Down No propagation Up Nopropagation Delete Grant Down No propagation Up No propagation Deny DownNo propagation Up No propagationThe propagation rules above are illustrative and not exclusive. Thoseskilled in the art recognize that different propagation rules can beimplemented, and that the method and system of the present invention isnot limited thereby.

In step 406, for each modified normalized rule, the translator 300applies the propagation rules and identifies each path that is affectedby the modified normalized rule. For example, if the modified normalizedrule associated with a node is a “GRANT read” to user A, the modifiednormalized rule propagates up and down from the node. User A has “read”access to all descendant nodes and to all ancestor element nodes. Thus,the translator 300 identifies the paths associated with the descendantand ancestor element nodes as being accessible by user A. If more thanone modified normalized rule affects a particular path, those rules arecombined for the path in step 406.

Next, in step 408, the translator 300 optimizes the rules by minimizingrepeated value expressions in the output. In a preferred embodiment,reference notations and supplemental value expressions are used tooptimize the rules. For the reference notation, the translator 300compares the ConditionID specified for a path associated with a node tothe ConditionID of the path associated with its parent node. If theConditionIDs are identical, then the ConditionID in the child node isreplaced with “ref(1, . . . /)” indicating that the condition for thechild node is identical to the condition of the parent node andtherefore, there is no need to reevaluate the value expression for thechild. In the case of a sibling reference, “ref(2, . . . /sibling-node)”is used to express the reference while “ . . . /sibling-node” is therelative path.

A supplemental value expression is an additional value expressionassociated with a path that describes the access rule for any descendantpath that exists or may exist in the future. For example, if user B hasread access on path “a/b,” then according to the propagation rulespresented in Table 1, user B has read access to any descendant pathassociated with a node (descendant path/node) under “a/b.” A descendantpath/node may exist (i.e., a path and value expression has beengenerated for the descendent node) or it may not yet exist because, forexample, a document containing this path has not yet been added to thecollection and processed by the Access Control mechanism 200. In thiscase, the translator 300 generates a supplemental value expression forpath “a/b” that indicates that user B has read access to any descendentpath/node. Thus, if and when a new descendant path/node is introduced,there is no need to generate a value expression for the new path.

Those skilled in the art will readily appreciate that utilizingreference notations and supplemental value expressions are but two waysto optimize the rules. Other techniques can be utilized to furtheroptimize the rules.

After optimization, a value expression generator 302 transforms eachmodified normalized access control rule into a value expression for thepath, via step 410. In a preferred embodiment, the value expressiongenerator 302 does two things: a syntactical conversion and an additionof a “!p” notation. For the syntactical conversion, each conditionexpression is transformed into syntax defined as XPath step qualifier.For example, a condition expression:

-   -   equal($Group, Admin) & xpath(/bib/book[@year=2000])        is transformed into:    -   [$Group=‘Admin’ and @year=2000] for the path “/bib/book”.        For the “!p” notation, the value expression generator 304        generates “!p” notation wherever the reference notation is        specified. For example, if ref(1, . . . /) is specified at        “/bib/book/title”, then the value expression is extended to “if        !p then [ref(1, . . . /)]”. When compiled and executed, the “!p”        notation indicates to the DBMS 105 that the value expression for        that path is the same as that for the path of the parent node        and therefore, the DBMS 105 does not need to reevaluate the        value expression if access to the parent has already been        granted. After step 410 is finished, a value expression is        generated for each path.

The above described value expression generation process can further bedescribed through the following simple example. Suppose the followingXML document is representative of a collection:

<bib ver=”1.0”>  <book year=“1994”>   <title>TCP/IP Illustrated</title>  <author>Alice</author>  </book>  <book year=“2002”>   <title>AdvancedProgramming in the Java environment</title>  </book> </bib>and the following rules make up the access control policy for thecollection:

Rule 1: </bib, {Murata}, +read> Rule 2: </bib/@ver, {Seki}, +read> Rule3: </bib/book, {Hada}, −read> Rule 4: </bib/book/title, {Tozawa}, +read>Rule Normalization (Step 402)

Here, rules 1-4 are transformed into normalized rules. Each normalizedrule includes a head, a path expression and a condition. The normalizedrules are as follows:

Rule 1 : (grant_read, /bib, equal($User, Murata)) Rule 2 : (grant_read,/bib/@ver, equal($User, Seki)) Rule 3 : (deny_read, /bib/book,equal($User, Hada)) Rule 4 : (grant_read, /bib/book/title, equal($User,Tozawa))Condition Table Generation (Step 404)

Next, the translator 300 generates and populates the following conditiontable:

ConditionID Condition Expression C1 equal($User, Murata) C2 equal($User,Seki) C3 equal($User, Hada) C4 equal($User, Tozawa)The translator 300 then converts the normalized rules into the followingmodified normalized rules:

Rule 1 : (grant_read, /bib, C1) Rule 2 : (grant_read, /bib/@ver, C2)Rule 3 : (deny_read, /bib/book, C3) Rule 4 : (grant_read,/bib/book/title, C4)Rule Propagation and Combination (Step 406):

Here, each rule is propagated through each path in the node tree(generated by the path generator 202 in step 306 of FIG. 3) according tothe propagation rules in Table 1 above. The output of the propagation ofthe rule through a path is a statement indicating whether a user isgranted or denied access to the path, and in turn, to the node. Thus,when each rule is propagated through each path making up therepresentative document, the resulting evaluation can be presented asfollows:

Rule 1 (grant_read, /bib, C1)     /bib : C1     /bib/@ver : C1    /bib/text( ) : C1     /bib/book : C1     /bib/book/text( ) : C1    /bib/book/@year : C1     /bib/book/title : C1    /bib/book/title/text( ) : C1 Rule 2 (grant_read, /bib/@ver, C2)    /bib : C1, C2     /bib/@ver : C1, C2     /bib/text( ) : C1    /bib/book : C1     /bib/book/text( ) : C1     /bib/book/@year : C1    /bib/book/title : C1     /bib/book/title/text( ) : C1 Rule 3(deny_read, /bib/book, C3)     /bib : C1, C2     /bib/@ver : C1, C2    /bib/text( ) : C1     /bib/book : C1, {C3}     /bib/book/text( ) :C1, {C3}     /bib/book/@year : C1, {C3}     /bib/book/title : C1, {C3}    /bib/book/title/text( ) : C1, {C3} Rule 4 (grant_read,/bib/book/title, C4)     /bib : C1, C2, C4     /bib/@ver : C1    /bib/text( ) : C1, C2     /bib/book : C1, C4, {C3}    /bib/book/text( ) : C1, {C3}     /bib/book/@year : C1, {C3}    /bib/book/title : C1, C4, {C3}     /bib/book/title/text( ) : C1, C4,{C3} Note: { } indicates that a condition inside the parenthesis impliesnegative permission.Optimization (Step 408)

Here, reference notations are used to minimize repetitive outputs.

/bib : C1|C2|C4 /bib/@ver : C1 /bib/text( ) : C1|C2 /bib/book :(C1|C4)&!C3 /bib/book/text( ) : C1&!C3 /bib/book/@year : C1&!C3/bib/book/title : ref(1, ../) /bib/book/title/text( ) : ref(1, ../../)Value Expression Generation (Step 410)

Here, each output statement is transformed into a value expression basedon the condition expression in the condition table. A value expressionis created for each path.

/bib : [$User=’Murata’ or $User=’Seki’ or   $User=’Tozawa’] /bib/@ver :[$User=’Murata’] /bib/text( ) : [$User=’Murata’ or $User=’Seki’]/bib/book : [($User=’Murata’ or $User=’Tozawa’) and   not($User=’Hada’)]/bib/book/text( ) : [$User=’Murata’ and not($User=’Hada’)]/bib/book/@year : [$User=’Murata’ and not($User=’Hada’)] /bib/book/title: if !p then [ref(1, ../)] /bib/book/title/text( ) : if !p then [ref(1,../../)]

Referring again to FIG. 3, after the value expressions have beengenerated, the Access Control mechanism 200 stores the paths andcorresponding value expressions in the path table 204 in step 310. FIG.6 illustrates a path table 600 for the collection of documents and theaccess control policy according to the example described above. As isshown, the table 600 comprises a path column 602 and a value expressioncolumn 604. The path column 602 contains the path to each node in thestructured document and the value expression column 604 contains thevalue expression. In a preferred embodiment of the present invention,the value expressions are compiled before they are stored in the pathtable 204. The result is a set of mini-plans that can be efficientlyexecuted by the DBMS 105 at runtime to perform access control checking.

To describe further how the value expressions are utilized by the DBMS105 during access control evaluation at runtime, please refer now toFIG. 7, which is a flowchart illustrating a process 700 for utilizingvalue expressions according to a preferred embodiment of the presentinvention. The process begins in step 702 when a user requests access,e.g., to read, to a node in a structured document in a collection. Therequest is typically in the form of a query that is written in Xquery orany other query language appropriate for querying structured documents.Typically, the DBMS 105 compiles the query during compile time and thenexecutes the compiled query during run time. The DBMS 105 returns thestructured document(s), or portions thereof, satisfying the query.

According to a preferred embodiment of the present invention, during theexecution of the query (during run time), the DBMS 105 performs anaccess control check to determine whether the user is authorized toaccess the requested node. The DBMS 105 does this by accessing the pathtable 204 in the Access Control mechanism 200 and evaluating the valueexpression corresponding to the path for the requested node in step 704.If the value expression indicates that access is granted to the user(step 706), the DBMS 105 returns the document with the requested dataviewable to the user in step 708. If, however, access is denied, theDBMS 105 returns the document with the requested data hidden from theuser in step 710. In step 712, a next requested node is evaluated andsteps 704 to 710 are repeated.

According to the preferred embodiment of the present invention, theAccess Control mechanism 200 of the present invention controls how thestructured document appears to the user when it is returned in a resultset. The data to which the user is granted access is displayed, whereasthat to which the user is denied access is hidden. Thus, path-levelaccess control prevents the user from gaining access to portions of adocument. For example, consider the representative document used in theexample above and the access control rules (Rules 1-4) associatedtherewith. A request from user Hada to read the document (/bib) wouldresult in the following output:

-   -   <bib ver=“1.0”>    -   </bib>.        The paths “/bib/book,” “/bib/book/text( ),” “/bib/book/@year,”        “/bib/book/title” and “/bib/book/title/text( )” are hidden from        user Hada because Rule 3 denies Hada read access to the path        “/bib/book.”

Through the aspects of the present invention, path-level access controlto a structured document in a collection is improved by transforming anaccess control policy for the collection into a set of valueexpressions. According to the preferred embodiment of the presentinvention, one value expression is generated for each path associatedwith a node in the structured document. The value expression is a simplestatement indicating who, if anyone, has access to the node associatedwith the path.

Thus, during access control evaluation, the DBMS 105 checks the valueexpression corresponding to the requested node/path, as opposed toevaluating the access control policy.

By evaluating value expressions instead of the access control policy,access control evaluation is more efficient and fast. Moreover, becauseaccess control evaluation is performed during run time, as opposed tocompile time, changes to security can be implemented without recompilingthe query. In addition, by performing access control evaluation duringrun time, the DBMS 105 is able to hide data in a document. Although thepresent invention has been described in accordance with the embodimentsshown, one of ordinary skill in the art will readily recognize thatthere could be variations to the embodiments and those variations wouldbe within the spirit and scope of the present invention. Accordingly,many modifications may be made by one of ordinary skill in the artwithout departing from the spirit and scope of the appended claims.

What is claimed is:
 1. A computer-implemented method for controllingaccess to structured documents, the method comprising: providing anaccess control policy for a structured document comprising a pluralityof nodes, wherein the access control policy comprises a plurality ofaccess control rules; generating a path for each of the plurality ofnodes in the structured document; and generating an executable valueexpression for each path by normalizing the plurality of access controlrules into a format comprising a head, a path, and a condition thatindicates who is granted or denied access to each path, generating andpopulating a condition table to convert the plurality of normalizedaccess control rules into a plurality of modified normalized accesscontrol rules, propagating the plurality of modified normalized accesscontrol rules for each generated path to identify at least one generatedpath that is affected by at least one of the plurality of modifiednormalized access control rules, combining at least two modifiednormalized access control rules of the plurality of modified normalizedaccess control rules if the at least two modified normalized accesscontrol rules affect a particular generated path, optimizing theplurality of modified normalized access control rules by eliminatingrepeated value expressions, and transforming the plurality of modifiednormalized access control rules into the executable value expression foreach path based on the condition, wherein the executable valueexpression is utilized during access control evaluation to determinewhether a user is allowed to access at least one of the plurality ofnodes in the structured document.
 2. The method of claim 1, furthercomprising: storing each path and corresponding generated valueexpression in a table.
 3. The method of claim 2, further comprising:prior to the storing, compiling each generated value expression.
 4. Themethod of claim 3, wherein the compiling further comprises: receiving aquery from the user, wherein the query requests access to a node in thestructured document; executing the query; evaluating the valueexpression corresponding to the path associated with the requested node;displaying data associated with the requested node if the valueexpression grants access to the user; and hiding data associated withthe requested node if the value expression denies access to the user. 5.The method of claim 4, wherein the evaluating is performed during a runtime.
 6. The method of claim 2, further comprising: replacing the valueexpression for a path associated with a node with a reference notationif the value expression is identical to that for a path associated withthe node's parent, thereby eliminating repeated value expressions in thetable.
 7. The method of claim 1, wherein the generating the valueexpression for each path further comprises: propagating each of theplurality of normalized access control rules through each path such thataccess to each path is defined by at least one normalized access controlrule; and transforming each of the at least one normalized accesscontrol rules affecting each path into a statement indicating who isgranted and denied access to the path.
 8. The method of claim 1, whereinthe providing further comprises: writing the plurality of access controlrules; and validating the plurality of access control rules such thatthe resulting rules are syntactically and logically valid.
 9. The methodof claim 1, wherein the structured document is written in ExtensibleMarkup Language.
 10. A non-transitory computer readable medium encodedwith a computer program for controlling access to structured documents,the computer program comprising instructions for: providing an accesscontrol policy for a structured document comprising a plurality ofnodes, wherein the access control policy comprises a plurality of accesscontrol rules; generating a path for each of the plurality of nodes inthe structured document; and generating an executable value expressionfor each path by normalizing the plurality of access control rules intoa format comprising a head, a path, and a condition that indicates whois granted or denied access to each path, generating and populating acondition table to convert the plurality of normalized access controlrules into a plurality of modified normalized access control rules,propagating the plurality of modified normalized access control rulesfor each generated path to identify at least one generated path that isaffected by at least one of the plurality of modified normalized accesscontrol rules, combining at least two modified normalized access controlrules of the plurality of modified normalized access control rules ifthe at least two modified normalized access control rules affect aparticular generated path, optimizing the plurality of modifiednormalized access control rules by eliminating repeated valueexpressions, and transforming the plurality of modified normalizedaccess control rules into the executable value expression for each pathbased on the condition, wherein the executable value expression isutilized during access control evaluation to determine whether a user isallowed to access at least one of the plurality of nodes in thestructured document.
 11. The computer readable medium of claim 10,further comprising computer program instructions for: storing each pathand corresponding generated value expression in a table.
 12. Thecomputer readable medium of claim 11, further comprising computerprogram instructions for: prior to the storing, compiling each generatedvalue expression.
 13. The computer readable medium of claim 12, furthercomprising computer program instructions for: receiving a query from theuser, wherein the query requests access to a node in the structureddocument; executing the query; evaluating the value expressioncorresponding to the path associated with the requested node; displayingdata associated with the requested node if the value expression grantsaccess to the user; and hiding data associated with the requested nodeif the value expression denies access to the user.
 14. The computerreadable medium of claim 13, wherein the evaluating is performed duringa run time.
 15. The computer readable medium of claim 11, furthercomprising computer program instructions for: replacing the valueexpression for a path associated with a node with a reference notationif the value expression is identical to that for a path associated withthe node's parent, thereby eliminating repeated value expressions in thetable.
 16. The computer readable medium of claim 10, wherein thecomputer program instructions for generating the value expression foreach path further comprises computer program instructions for:propagating each of the plurality of normalized access control rulesthrough each path such that access to each path is defined by at leastone normalized access control rule; and transforming each of the atleast one normalized access control rules affecting each path into astatement indicating who is granted and denied access to the path. 17.The computer readable medium of claim 10, wherein the computer programinstructions for providing further comprises computer programinstructions for: writing the plurality of access control rules; andvalidating the plurality of access control rules such that the resultingrules are syntactically and logically valid.
 18. The computer readablemedium of claim 10, wherein the structured document is written inExtensible Markup Language.
 19. A computer system for controlling accessto structured documents, the computer system comprising: a databasemanagement system implemented on the computer system; the datamanagement system including a processor for executing instructions onthe computer system, the database management system comprising: anaccess control policy for a structured document, wherein the structureddocument comprises a plurality of nodes and the access control policycomprises a plurality of access control rules, and an access controlmechanism configured to: generate a path for each of the plurality ofnodes in the structured document and generate an executable valueexpression for each path by normalizing the plurality of access controlrules into a format comprising a head, a path, and a condition thatindicates who is granted or denied access to each path, generating andpopulating a condition table to convert the plurality of normalizedaccess control rules into a plurality of modified normalized accesscontrol rules, propagating the plurality of modified normalized accesscontrol rules for each generated path to identify at least one generatedpath that is affected by at least one of the plurality of modifiednormalized access control rules, combining at least two modifiednormalized access control rules of the plurality of modified normalizedaccess control rules if the at least two modified normalized accesscontrol rules affect a particular generated path, optimizing theplurality of modified normalized access control rules by eliminatingrepeated value expressions, and transforming the plurality of modifiednormalized access control rules into the executable value expression foreach path based on the condition, wherein the executable valueexpression is utilized during access control evaluation to determinewhether a user is allowed to access at least one of the plurality ofnodes in the structured document.
 20. The computer system of claim 19,wherein the access control mechanism is configured to store each pathand corresponding generated value expression in a table.
 21. Thecomputer system of claim 20, wherein the database management systemfurther comprises a compiler configured to compile each generated valueexpression prior to the storage.
 22. The computer system of claim 21,wherein the compiler further receives a query from the user, wherein thequery requests access to a node in the structured document, executes thequery, evaluates the value expression corresponding to the pathassociated with the requested node, displays data associated with therequested node if the value expression grants access to the user, andhides data associated with the requested node if the value expressiondenies access to the user.
 23. The computer system of claim 22, whereinaccess control evaluation is performed during a run time.
 24. Thecomputer system of claim 19, wherein the access control mechanismcomprises: a translator propagating each of the plurality of accesscontrol rules through each path such that access to each path is definedby at least one access control rule; and a value expression generatorfor transforming each of the at least one access control rulesassociated with each path into a statement indicating who is granted anddenied access to the path.
 25. The computer system of claim 19, whereinthe access control rules are syntactically and logically valid.
 26. Thecomputer system of claim 19, wherein the structured document is writtenin Extensible Markup Language.